The present invention relates to methods for automatically controlling the amplitude modulation of video signals. Both the absolute carrier level and the depth of modulation are controlled. The invention is suitable for use with unscrambled television signals as well as television signals that have the horizontal synchronization pulse suppressed to prevent detection of the signals by unauthorized persons, e.g., pirates.
A composite video signal such as one which conforms to the NTSC standard includes picture luminance and chrominance information as well as timing information for the synchronization of scanning and color processing circuits at a receiver. At the end of each line scan at the receiver, a horizontal synchronizing pulse (HSYNC) commands the scanning circuit to return the scanning beam to the left of the screen to begin scanning a new line. Similarly, at the completion of each field or frame, a vertical synchronizing pulse (VSYNC) commands the scanning circuit to return to the top of the screen to begin scanning the next field or frame. The return period is known as the vertical blanking interval.
Accordingly, the television signal may be scrambled by altering the normal position and/or amplitude of the synchronization pulses. Such techniques for scrambling the video portions of television signals are well known. For example, U.S. Pat. No. 3,813,482 to Blonder discloses a system for transmitting television signals where the video is scrambled by suppressing the vertical or horizontal synchronization pulses to produce a shifting or rolling scrambled picture. U.S. Pat. No. 4,542,407 to Cooper et al, discloses an apparatus for scrambling and descrambling television programs in which the horizontal synchronization information is suppressed at a cable television (CATV) headend, and then regenerated by a subscriber's cable television converter. U.S. Pat. No. 4,095,258 to Sperber, U.S. Pat. No. 4,163,252 to Mistry et al., and U.S. Pat. No. 4,571,615 to Robbins et al. describe schemes for decoding scrambled television signals.
In particular, by suppressing the horizontal synchronization pulses below the average value of the active video, a television receiver attempts unsuccessfully to lock horizontally on to random peaks of the active video rather than the HSYNC pulses. Additionally, the loss of effective horizontal synchronization prevents the receiver from properly utilizing the color burst signal which is associated with the HSYNC pulse, so that color reproduction is also faulty.
In order for a receiver to restore (i.e., descramble) the scrambled video signal, the suppressed synchronization pulses must be restored. This may be accomplished by amplitude modulating timing pulses on the FM audio carrier of the television signal. The pulses are then detected in the audio portion of the receiver and used to generate the timing signals necessary to descramble the received video signal. Alternatively, a portion of the sync timing pulses is transmitted without suppression, for example, during the vertical blanking interval. The receiver is phase-locked to the unsuppressed signals to create the required timing and synchronization information for descrambling the video portion of the signal.
Further scrambling of a television signal may be achieved by inverting a portion of the active video such as described in U.S. Pat. No. 4,598,318 to Robbins.
Generally, amplitude modulation of the active video is achieved by varying the amplitude of an RF carrier signal about a fixed level. The variation in amplitude is known as the depth of modulation. Additionally, synchronization signals can be suppressed to −6 dB or −10 dB below the unsuppressed level which is specified by the transmission standard which is used, e.g., −40 IRE for NTSC signals. The suppression level may be varied with time, for example, according to detected scene changes in the active video. Switching may occur several times per second, or more slowly, such as once every several seconds. Conventionally, a stable amplitude modulator is used with automatic gain control of the video signal.
However, suppression and subsequent restoration of synchronization pulses is hampered by variations in amplitude modulation equipment, which may be at a CATV headed or a remote location, for example. Conventional RF carrier circuitry is subject to drift and other inaccuracies due to humidity and temperature variations, as well as changes due to degradation over the lifetime of the equipment, for example. Modulation level discrepancies can cause flickering or other undesirable brightness changes in the recovered video image.
Thus, such modulating circuitry must be periodically adjusted by a technician using metering equipment to ensure accuracy. This solution is inefficient, in particular, when the modulation circuitry is remotely located. Additionally, the problem of drift in the modulation accuracy is not solved.
Accordingly, it would be desirable to have a system for automatically controlling the amplitude modulation level and depth of modulation of a television signal. The system should be suitable for use with sync suppressed signals, including signals with VSYNC and/or HSYNC suppression, as well as non-suppressed signals. The system should be suitable for use with multiple levels of sync suppression. The system should also be suitable for use with signals with normal (e.g., non-inverted) as well as inverted active video portions.
The system should further be relatively inexpensive to manufacture and install, and should require only a low-speed microprocessor controller. The system should provide an assembly with a common microprocessor controller which services a number of individual modulation circuits on a time-sharing basis.
The present invention provides a system having the above and other advantages.